Class Hydrozoa

Hydrozoans are a diverse group of mostly marine cnidarians that include fire corals, hydroids, and the Portuguese man o' war. They are mainly characterized by their lifestyle, which alternates between polyp and medusa stages. Many species form colonies of polyps called zooids, and altogether they function as a single organism. Unlike cubozoans and scyphozoans, their medusae are small and have a thin membrane called a velum, helping them swim. They exhibit radial symmetry, and their tentacles are aligned with cnidocytes, which contain nematocysts for capturing plankton. Their mouth is encircled by their tentacles, and their gastrovascular cavity has a network of canals that distribute nutrients. Some hydrozoans lack a medusa stage completely and only exist as solitary polyps, while some colonial species have specialized zooids that aid in feeding, defense, and reproduction.